1. Field of the Invention
The present invention relates to power transmission apparatus provided with differential limiting function, and more specifically to a power transmission apparatus which can limit and also promote differential operation between two transmission shafts.
2. Description of the Prior Art
Conventionally, the power transmission apparatus provided with differential limiting function is disclosed in Japanese Unexamined Published (Kokai) Patent Application No. 62-214021, for instance, in which power is transmitted by multiplying differential rotation produced between two input and output shafts through a planetary gear device, in order to restrict the differential rotation by a centrifugal brake.
In addition, Japanese Unexamined Published (Kokai) patent Application Nos. 56-43031 and 55-72420 disclose power transmission apparatus of wet multidisc clutch type, in which differential distribution and differential operation are restricted by a hydraulic clutch; Japanese Examined Published (Kokoku) Patent Application No. 50-53 discloses an apparatus in which a cam mechanism and a multidisc clutch are used as resistance against differential rotation; Japanese Unexamined Published (Kokai) Patent Application No. 62-165032 discloses an apparatus of viscous coupling type; further, Japanese Unexamined Published (Kokai) Patent Application No. 60-104426 and Japanese Examined Published (Kokoku) Utility Model Application No. 46-35772 disclose an apparatus using a pump, respectively. In these power transmission apparatus, however, differential function is controlled so that difference in rotation speed between the two shafts can only be reduced.
As described in more detail later, vehicles are often required to promote differential rotation between vehicle wheels, according to vehicle travelling, steering or road surface conditions, in addition to differential rotation restriction. In the prior-art power transmission apparatus of differential control type, however, since differential rotation is restricted only in the decreasing direction, there inevitably exists a limit when vehicle travelling performance is required to improve.